In the field of vaccination, first generation vaccines contained only the antigen against which an immune response was desired. However, because the presence of an antigen alone is in most cases only weakly efficient, a second generation of vaccines was developed, where the vaccinating composition included one or more adjuvants as immunomodulators (i.e., GM-CSF alone or in combination with other adjuvants) to enhance this immune response. In order to be effective, the adjuvant must be stably released at the vaccination site for several days.
Several different techniques have been reported for providing the adjuvant at the vaccination site, and the choice of the technique depends on the context of the immunization.
For example, in the context of antigen-based vaccines (as opposed to cell-based vaccines), a widely applicable technique is simply to combine the antigen with the adjuvant in the vaccinating composition. The resulting composition is administered directly to the subject, thereby supplying the antigen and adjuvant in a simultaneous and co-localized manner.
However, this simple approach cannot be used in all vaccination settings. For example, in most cancers (e.g., lung, colon, stomach, lymphoma, and brain), useful antigens for vaccination are often not known. Therefore, for these types of cancer, cell-based immunization strategies against tumors are needed. For immunization strategies involving cell-based vaccines, the antigen(s) is produced by whole cells, which are implanted in a subject. Such strategies require the use of more elaborate techniques to ensure the efficient delivery of the adjuvant.
In one example, the immunomodulator is directly injected at the vaccination site, either in a “naked” form or in a slow release formulation using pegylated, liposomal microspheres. However, this strategy is often limited by technical and biochemical difficulties, as systemic administration of the adjuvant is not efficient and may be toxic and local release of recombinant proteins being used as an adjuvant (i.e., GM-CSF) is not reliable because GM-CSF is an unstable protein that has a half-life of only a few hours within the human body. Thus, in order to be effective, GM-CSF has to be continuously produced in situ in order to be therapeutically effective.
Another example used to circumvent the problems arising from the direct injection technique is the use of “bystander cells” to locally produce the immunomodulators. In these methods, cells producing the required adjuvant are implanted in proximity to the source of the antigen, thereby providing an efficient, local release of adjuvant at the vaccine site.
However, this approach also has some drawbacks. For human immunization, multiple immunizations are required, and, because syngeneic bystander cells are not readily available, allogenic cells are most often used. Thus, after the first injection, the bystander cells are recognized by the immune system of the host (allorecognition) and are rejected, thereby preventing further stable and sustained production of immunomodulator and jeopardizing the desired immune response against the antigenic substance of the vaccine.
In order to overcome this allorecognition problem, Borrello et al (Human Gene Therapy, 1999, 10(12), 1983-1991) described a strategy in which the GM-CSF-supplying cell is a cell line, K-562 (ATCC Deposit No. CCL-243), which do not express MCH molecules on their surface and fail to express HLA class I or II antigens, thereby potentially decreasing the magnitude of the alloresponses generated on repeated immunizations. However, these cells are human cancer cells and are highly sensitive to potent rejection mechanisms that occur without the involvement of HLA class I or II proteins that are less specific but are very rapid and potent for cellular destruction. For example, K-562 cells are known to be very sensitive to NK cells and also to γδ T cells leading to rapid elimination of allogeneic cells.
Therefore, it is likely that K-562 bystander cells injected at the vaccine site will be destroyed efficiently and quickly by non-MHC dependent cytotoxic mechanisms, which may significantly decrease the release of the immunomodulator.
Moreover, in addition to being very sensitive to rapid destruction by NK cells, K-562 cells can also express MHC class I upon interferon γ exposure. Because interferon γ could be present or released at the vaccination site during the first or after repeated immunizations, such MHC class I upregulation will also lead to rapid cell destruction via classical cellular immunity.
For these reasons, use of cells such as K-562 in vaccination is associated with numerous drawbacks.
Another solution that is widely used in the context of cell-based vaccines is to couple the production of antigen and the release of immunomodulator by engineering the cell that is the source of antigen to also supply the immunomodulator. For example, in cancer vaccines, the source of antigen is usually a whole tumor cell, which can be engineered, for example by transfection, to simultaneously produce the necessary adjuvant.
In view of the favorable results obtained in the mouse model, the initial human trials used the same strategy. However, the technique proved to be very labor intensive and time consuming because the patient's surgically harvested cells need to be expanded in vitro for retroviral infection, thereby preventing a wide use of the method.
The use of other viral vectors to infect the tumor cells has also been proposed to circumvent the difficulties observed with the use of retroviral vectors.
Nevertheless, the major problem associated with the new viruses tested is that, in most cases, some viral proteins will be expressed from the tumor cells after infection, and these viral proteins are strongly recognized by the immune system as foreign, infectious agents. Therefore, the initial goal of mounting an immune response against weak tumor antigens is skewed or diverted towards a viral protein, which results in masking the anti-tumor immune response and priming the recipient against subsequent immunization, which will further increase the destruction of the injected cells and will likely decrease the efficacy of the anti-tumor immunization scheme.
Thus, while the use of autologous engineered tumor cells as combined source of antigen and adjuvant a priori minimizes the risk of undesirable immune response, the step of viral infection itself gives rise to significant problems.
In order to limit the problems arising from viral infection of autologous cells, new strategies have been developed which do not require the patients' cells. In these techniques, the antigenic source is provided by cell lines derived from other patients with similar type of cancer, and the patient is immunized with repeated injections of irradiated, GM-CSF secreting, allogeneic (from another human being) tumor cells. The percentage of patients showing an immune response in studies using these techniques has been lower than expected.
Accordingly, there remains a need in the art to develop vaccine compositions that provide both a constant source of immunomodulator and an antigenic component that is substantially free of undesirable interactions with the natural or adaptative immune system.